1. Field of the Invention
The present invention relates to an injection molding method for carrying out injection molding so as to allow a mixed resin formed when a variety is changed present within a molded product, and a molding machine.
2. Description of the Related Art
In the injection molding machine, generally, as shown in FIG. 16, resins are supplied from a hopper 52 into a cylinder barrel 51, a screw 53 is rotated to deliver the resins forward while plasticizing the latter, and the screw 53 is moved back. When a fixed quantity of resins are measured according to a position of the screw, the rotation of the screw 53 is stopped and moved forward whereby the resins measured are extruded from a nozzle part 51a into a metal mold not shown to carry out injection molding.
However, where the variety of the molded product is changed as in changing color, it is necessary that the resins within the cylinder barrel 51 be completely replaced with new resins so as not to be affected by the resins before changed. Accordingly, in the past, from a viewpoint of workability and productivity, conventionally, new resins are supplied through the hopper 52, and the screw 53 is rotated and/or moved forward and backward so that the resins before changed are extruded by new resins and discharged outside the machine. When the resins before changed are completely discharged outside the machine, by replacement such an operation as described above, replacing is completed and injection molding for next molded products is carried out.
However, in the above-described method, it is necessary that a large quantity of resins from 10 to 100 shots are discharged outside the machine till the resins within the cylinder barrel 51 are completely replaced. Since the resins to be discharged outside the machine are in the state of mixed resins in which new and old resins are mixed so that colors are mixed, when they are used as materials for molded products, an inconvenience such as a color mixture appears on the surface of the molded products. Therefore, in the past, a large quantity of resins (mixed resins) are to be abandoned taking much time every change of the variety of molded products such as color changing, resulting in an increase of cost of molded products due to an increase in material loss and a lowering of the working rate of the machine in actual molding.
It is an object of the present invention to provide an injection molding method and a molding machine wherein even where mixed resins formed when resins are replaced are used as materials for molding products, no inconvenience appears on the surface of the molded products, and molding using the mixed resins can be carried out together with the operation of changing the resins.
The injection molding method according to the present invention selectively carries out a normal molding operation in which injection units are connected together which measures resins while plasticizing the latter and injects into a cavity part, and only desired resins in one of the injection units are injected into the cavity part to thereby mold a single layer molded product; and a multilayer molding operation in which mixed resins formed when resins are changed in one of the injection units are injected into the cavity part so as to constitute a core layer, and desired resins in the other of the injection units are injected into the cavity part so as to constitute a skin layer thereby molding a molded product of a 3-layer construction.
In this case, when the mixed resins are formed due to the changing of resins, the mixed resins are transferred to the multilayer molding operation for molding the molded products of the 3-layer construction using the core layer to thereby enable obtainment of molded products having the surface of the skin layer comprising desired resins without appearance of the inconvenience resulting from the mixed resin on the surface of the molded products. Since this molding of molded products is carried out by injecting the mixed resins to discharge the latter into the core layer of the molded product, that molding can be carried out together with the operation of changing resins. Thereby, it is possible to obtain molded products using the mixed resins in conjunction with the operation of changing resins, and therefore, it is possible to obtain molded products at less cost by the reduction in material loss and the enhancement of the working rate of the machine in the actual molding, as compared with the conventional case where a large quantity of resins (mixed resins) are abandoned taking much time.
The above-described multilayer molding operation comprises enabling supplying and receiving resins between the injection units plasticizing the mixed resins in one of the injection units and measuring the latter so as to have a shot part corresponding to the core layer, supplying, after the desired resins have been plasticized in the other of the injection units, the resins to one of the, injection units so as to have a shot part corresponding to the skin layer, and injecting the desired resins and the mixed resins into the cavity part in said order from one of the injection units to thereby mold the molded products of the 3-layer construction.
In this case, when the desired resins and the mixed resins are injected into the cavity part in said order, the desired resins injected first are cooled from the wall surface of the cavity part and solidified. Accordingly, when the mixed resins are then injected, the mixed resins flow along the inside part corresponding to the soft core layer of the desired resins whereby the desired resins are press fitted deeply into the cavity part in the state that the desired resins contain the mixed resins therein. Accordingly, when the total quantity of the mixed resins are injected into the cavity part, there assumes a 3-layer construction in which the desired resins are present on both sides of the mixed resins, and therefore, it is possible to obtain molded products of a 3-layer construction in which the mixed resins of the core layer are covered with the desired resins of the skin layer. According to the above-described injection molding method, it is possible to obtain the molded products of a 3-layer construction without connecting the injection units together using a special mechanism, thus reducing the cost of apparatus.
Further, the aforementioned multilayer molding operation comprises connecting the injection units so as to be capable of changing into a flow path for a core layer used for forming a core layer and a flow path for a skin layer used for forming a skin layer, measuring the mixed resins so as to have a shot part corresponding to the core layer while plasticizing the mixed resins in one of the injection units, connecting the injection unit to the flow path for a core layer, measuring the desired resins so as to have a shot part corresponding to the skin layer while plasticizing the desired resins in the other of the injection units, connecting the injection unit to the flow path for a skin layer, and injecting the mixed resins in one of the injection units and the desired resins in the other of the injection units into the cavity part through the flow path for a core layer and the flow path for a skin layer, respectively, to thereby mold the molded products of a 3-layer construction.
In this case, by the simple operation that the connecting state of the injection units is changed relative to the flow path for a core layer and the flow path for a skin layer, it is possible to easily obtain the molded products of a 3-layer construction with the mixed resins in one of the injection unit being a core layer.
By the above-described injection molding method, the molded products are formed so as to have the mixed resins as a core layer, and the molded products are formed so as to have the mixed resins as a core layer in the range of 5 to 65% relative to the total weight.
Next, the injection molding machine according to the present invention for realizing the aforementioned injection molding method comprises an injection unit connecting device for connecting injection units together for measuring resins while plasticizing the latter to inject them into a cavity part; and a control device for selectively carrying out a normal molding operation for injecting only the desired resins in one of the injection units into the cavity part to thereby mold a single layer molding product, and a multilayer molding operation for injecting the desired resins in the other of the injection units into the cavity part so as to be a skin layer to thereby mold molded products of a 3-layer construction, wherein
the injection unit connecting device comprises a flow path for a core layer used for forming the core layer, a flow path for a skin layer used for forming the skin layer, and a switching mechanism for connecting the injection units capable of changing to either the flow path for a core layer or the flow path for a skin layer, and
the control device controls the switching mechanism so that in the multilayer molding operation, one of the injection units is connected to the flow path for a core layer, and the other of the injection units is connected to the flow path for a skin layer.
In this case, the molded products of a 3-layer construction with the mixed resin in one of the injection units being a core layer can be easily obtained by the simple constitution.